Introduction Simultaneous recording of EEG during transcranial electrical stimulation (tES) is a non-invasive and painless method to evaluate the modulation of cortical oscillatory brain activity and cerebral plasticity. Latest investigations in neuroscience with transcranial direct current (tDCS), alternating current (tACS) or random noise current (tRNS) provide a wide area of research topics ( Miniussi et al., 2012 , Thut et al., 2011 ). The distortion of the EEG signals during tES makes it impossible to evaluate the EEG signals. Therefore methods for artifact correction are necessary to reconstruct the underlying EEG signal during tES. Objectives The objective of this paper is to demonstrate a method for online artifact correction of EEG signals during tES. The measured distorted EEG signals are modeled as a linear superposition of the current artifact signals and raw EEG. A linear regression model can be used for subtraction of linear scaled current artifacts from the measured EEG. Therefore, an independent reference signal from the electrical stimulator has to be recorded simultaneously with the EEG signals. Materials and methods Two low noise, analogue signals were derived from the electrical stimulator (DC Stimulator PLUS, neuroConn GmbH, Ilmenau) as reference signals. The galvanic isolated signals (amplitude ± 40 mV) were fed into the EEG amplifier (full-band DC-EEG amplifier NEURO PRAX ®) together with EEG, EOG and ECG signals. The galvanic isolation of the reference signals ensures the electrical safety for the patient. Sinusoidal stimulation in the alpha-band of the EEG (10 Hz) was performed over P3 and P4 locations using easycap’s Ag/AgCl ring electrodes (current peak-to-peak amplitude: 1000 uA). A short-time learning period (about 10 s) was used to calculate the scaling factor of the regression model. Online correction was tested with eyes-closed condition (generation of alpha-waves). Results Fig. 1 shows the uncorrected EEG, EOG, ECG signals and the two reference signals (tACs1, tACs2). Beside the ECG, all EEG and EOG signals are strongly distorted by current artifacts. By comparing the EEG signals at one time-stamp (e.g. 12:16:22), the current artifacts (amplitude from 5000 uV to 30,000 uV) show different phase and polarity between the EEG channels. This has to be considered by the online correction method. Fig. 2 shows the same time window of all corrected signals (compare the ECG signals and time stamps of ( Fig. 1 , Fig. 2 ). Blinking eyes artifacts, DC shifts of frontopolar signals as well as alpha-EEG are clearly visible in the EEG signals during eyes-closed condition. All signal parts which are phase-synchronized to the 10 Hz sinusoidal stimulation, will be eliminated by the correction procedure. However, alpha-waves are still visible in T4, Pz, O1 and O2. Conclusions Artifacts in EEG signals during single-channel tES can be easily corrected using reference signals from the tES device. This new method was implemented into neurConn’s tES-EEG products.